High-resolution imaging of celestial objects through the atmosphere and the interstellar medium requires that the wavefront distortion caused by these media be removed before the light from the object reaches the imaging sensor. Distortion caused by a time-varying, inhomogeneous medium can be removed with one or more active optical components. In a typical configuration, light from a bright star near the target object is collected by a telescope, collimated, reflected by a flat deformable mirror, and passed through a lenslet array which produces a pattern of bright spots on an imaging CCD array. Any distortion in the light from the star causes image spots on the imaging array to shift in a characteristic way, allowing a computer receiving data from the CCD array to determine the necessary changes to the wavefront to restore it to the original shape before distortion. To accomplish the wavefront correction, the computer sends commands to an array of actuators that push or pull on a deformable mirror so that the distorted mirror surface compensates for the distortion in the received wavefront. The wavefront from the bright star is repeatedly analyzed and, in response, the shape of the deformable mirror is repeatedly adjusted. An imaging sensor images the full field-of-view of the telescope during a long exposure which may include multiple updates to the mirror surface. Under favorable conditions, a high-performance adaptive-optics system equipped with a deformable mirror can significantly improve the resolution of a ground-based telescope so that images of distant objects are resolved at resolutions similar to those achievable for imaging outside the atmosphere, such as for an on-orbit telescope system.
The design and fabrication of flexible mirrors remains one of the greatest challenges in the field of astronomical-telescope engineering, due in large part to the conflict between the rigidity necessary for the mirror to hold a precise optical surface and the flexibility required to perform wavefront correction. Deformable mirrors such as those used in high-performance adaptive-optics systems of major observatories are difficult to fabricate and expensive. Deformable mirrors typically are small. Moreover, difficulties associated with fabricating curved deformable mirrors generally means that adaptive-optics systems use a flat deformable mirror as an additional optical component in the optical path of a static imaging system. The additional optical surface of the deformable mirror increases the scattered light and absorptive light-loss, resulting in a decrease in the system throughput and contrast. In addition, the full telescope aperture is mapped onto the smaller aperture of the deformable mirror, effectively decreasing the transverse coherence length. Individual actuators must be spaced more closely as the size of the deformable mirror decreases to maintain the resolution of the wavefront correction, while the actuator stroke must continue to meet or exceed the required correction amplitude. The stiffness of deformable mirrors made from solid substrates limits the actuator density-stroke product, making the fabrication of mirrors with large strokes and high actuator densities challenging.
What is needed is a deformable mirror that overcomes the problems described above. The present invention satisfies this need and provides additional advantages.